Antimony plating bath



UnitedStates Patent ANTIMONY PLATING BATH Clarence F. Smart, Birmingham, Mich., assignor to General Motors Corporation, Detroit, Mich., a corporation of Delaware No Drawing. Application September 22, 1952, Serial No. 310,928

7 Claims. (Cl. 204-45) This invention relates to improvements in the electrodeposition of antimony.

The electrodeposition of antimony heretofore has resulted in dull antimony coatings which generally have required butfing or polishing to produce a satisfactory bright antimony surface. 7

I have now discovered that a lustrous, smooth, adherent and corrosion resistant coating of antimony which requires little or no buffing or polishing may be electrodeposited from an aqueous bath containing certain addition agents. According to the present invention, the preferred addition agents are boric acid, fluoboric acid or soluble salts of fluoboric acid. Illustrative of fiuoboric acid salts suitable as addition agents according to my invention are sodium fluoborate, ammonium fluoborate and ferrous fluoborate. While the sodium fiuoborate is the preferred salt of fluoboric acid, in general, any soluble fluoborate may be used. The addition agents of my invention may be used separately or, if desired,.several may be combined in one bath in the proper proportions.

Just why the addition to a plating bath of either the boric acid radical (B03) or the fluoboric radical (BF4) should permit the plating of a lustrous, adherent antimony coating is not known. However, these addition agents, added in amounts much greater than would be a conventional brightening agent are thought not only to act as brighteners but also to affect the ionization of the dissolved antimony.

While very beneficial results have been obtained in the electrodeposition of antimony from an aqueous bath consisting essentially of dissolved antimony and one of my addition agents, most satisfactory results have been obtained when the bath also contains an additional material such as dibasic ammonium citrate or acid sodium tartrate to promote anode corrosion. Other tartrates, citrates or acetates may also be used as anode corrosion promoters. If desired, citric acid and ammonium hydroxide or tartaric acid and sodium hydroxide may be employed in the proper amounts in the bath in lieu of ammonium citrate or sodium acid tartrate.

The use of my addition agents in antimony plating baths is illustrated but not limited by the following examples in which the quantities stated are per liter of water.

Example I A specific example of an antimony bath composition utilizing boric acid as an addition agent is the following:

60 to 100 grams of ammonium citrate (dibasic),

8 to 12 grams of antimony potassium tartrate (tartar emetic),

8 to 15 grams of boric acid.

Example II A specific example of an antimony bath composition utilizing a fluoborate as an addition agent is the following:

2,779,725 Ce Patented Jan. 29, 1957 60 to 100 grams of acid sodium tartrate, 8 to 12 grams of antimony chloride, 8 to 15 grams of sodium fiuoborate.

Baths of these compositions are very stable, have a high throwing power and provide excellent anode corrosion using antimony anodes. It will be understood of course that the antimony salt need not be restricted to antimony chloride or antimony potassium tartrate (tartar emetic). Any antimony salt soluble in the bath such as an antimony oxide for example, may be used as a source of antimony.

While the amounts of the various ingredients may be varied as indicated, it is generally desirable to maintain a ratio of ammonium citrate (dibasic), or acid sodium tartrate, or other anode corrosion promoter to the dissolved antimony of at least about 6 to 1 because many antimony salts when mixed with water form oxy salts which are generally insoluble in water. However, these salts dissolve readily in a solution having a large proportion of tartrate, acetate, citrate, etc. present.

In general, it is desirable to maintain enough addition agent in the bath to provide a ratio of addition agent to dissolved antimony ranging from 0.5 to 4.0. This ratio is critical only on the lower side, i. e. if the amount of addition agent in the bath falls much below half the amount of dissolved antimony present, the brightness of the electrodeposited antimony will tend to be reduced.

Various colloidal materials may be added in small amounts as anti-pitting agents to any baths having the above compositions. A preferred anti-pitting agent is animal glue added in an amount not exceeding 0.1 grams per liter and preferably in an amount of about 0.05 grams per liter.

Antimony may be electrodeposited from baths, having compositions as illustrated above, most satisfactorily in a pH range of from 4.5 to 5.5. The preferred pH is 5. By dissolving dibasic ammonium citrate in water according to the presently preferred method, a solution having a pH of approximately 5 is obtainedso that the preferred pH of the bath may be originally established by dissolving the dibasic ammonium citrate. The pH of the bath, of course, may be adjusted as necessary by additions of acids such as hydrochloric, nitric, acetic or sulfuric.

While a bath similar to those mentioned above may be operated at varying voltages and current densities, most satisfactory results have been obtained when a voltage of from 2 to 4 volts, 3 preferred, is used. While satisfactory results have been obtained when the current density varies from 15 to 25 amperes per square foot, the preferred current density is about 20 amperes per square foot.

A bath employing an addition agent of my invention may be operated at any desired temperature, limited generally only by the practical consideration of minimizing heating costs and evaporation losses. While increasing the temperature of the baths will increase the speed of plating by making the solution more electroconductive, it is not necessary to heat the bath to obtain a bright antimony electrodeposit since very satisfactory results have been obtained according to my invention when the bath is operated at about room temperature, i. e. 70 to F.

If it is desired to operate a plating bath embodying my invention using inert anodes formed of materials such as carbon or stainless steel and maintaining the concentration of antimony in the bath by separate additions of soluble antimony salts, then the ammonium citrate, acid sodium tartrate or any other anode corrosion promoter used, can be omitted entirely without affecting the high luster of the resulting electrodeposited coatings of antimony.

In preparing a bath employing the addition agents of my invention, the various ingredients may be mixed in any desired sequence. However, since some antimony salts, as explained above, are much more soluble in a citrate or tartrate solution than in water alone, it may be desirable at times to first dissolve the citrate or tartrate in water before adding the antimony salt.

In using the plating baths of this invention antimony may be electrodeposited on any conventional metal as a cathode such as lead, indium, alloys of lead and indium, copper, iron and steel, zinc, tin, cadmium, lead antimony alloys or antimony lead alloys.

In the preparation of lustrous, corrosion resistant antimony coatings the following is a present preferred sequence of plating. A metallic article is first coated with lead, indium or a lead-indium alloy to protect the article from corrosion in subsequent plating baths. This protective coating preferably has a bright smooth surface obtained either in its electrodeposition or by buffing or polishing. A layer of antimony is then electrodeposited over the protective coating-from a bath employing one of my addition agents to provide a corrosion resistant surface which may serve either as an attractive, protective exterior coating for the plated article or as an undercoat for further electrodeposited coatings of metals such as nickel or chromium.

It is to be understood that, although the invention has been described with specific reference to particular embodiments thereof, it is not to be so limited since changes and alterations therein may be made which are within the full intended scope of this invention as defined by the appended claims.

I claim:

1. An antimony .plating bath consisting essentially of an aqueous solution of dibasic ammonium citrate, an-

a, to grams per liter of dibasi e ammonium citrate, 8 to 12 grams per liter of antimony potassium t-ar-trate, and 8 to 15 grams per liter of boric acid.

3. An antimony plating bath consisting essentially of 60 to 100 grams per liter of dibasic ammonium citrate, 8 to 12 grams per liter of antimony potassium tartrate, and 8 to 15 grams per liter of fluoboric acid.

4. An antimony plating bath consisting essentially of 60 to 100 grams per'liter of dibasic ammonium citrate, 8 to 12 grams per liter of antimony potassium tartrate, and 8 to 15 grams per liter of a soluble salt of fiuoboric acid.

5. An antimony plating bath according to claim 4 in which the soluble salt of fiuoboric acid is sodium fluoborate.

6. A method of eletrodepositing antimony which comprises passing electric current from an anode to a cathode through the electrolyte defined in claim 1.

7. An antimony plating bath consisting essentially of an aqueous solution of dibasic ammonium citrate, antimony potassium tartrate and a compound selected from the group consisting of boric acid, fluoboric acid and soluble salts of fluoboric acid, the concentrations of the several ingredients per liter of water being within the following ranges: 60 to 100 grams of dibasic ammonium citrate, 8 to 12 grams of antimony potassium tartrate, and 8 to 15 grams of the compound selected from said group, the ratio of the amount of dibasic ammonium citrate to antimony in solution being at least 6 to l and the ratio of the amount of the compound selected from the said group to the amount of antimony being at least 0.5.

References Cited in the file of this patent UNITED STATES PATENTS 2,458,827 Booe Jan. 11, 1949 2,461,350 Schaefer et al. Feb. 8, 1949 2,634,235 Hitchens et a1. Apr. 7, 1953 OTHER REFERENCES Piontelli et al.: Chemical Abstracts, vol. 37 (1943), p. 1336.

Soderberg eta1.: Plating, vol. 37 (1950), pp. 254-259. Mathers et al.: Transactions Electrochemical Society, vol. 31 (1917), pp. 293-301. 

1. AN ANTIMONY PLATING BATH CONSISTING ESSENTIALLY OF AN AQUEOUS SOLUTION OF DIBASIC AMMONIUM CITRATE, ANTIMONY POTASSIUM TARTRATE AND A COMPOUND SELECTED FROM THE GROUP CONSISTING OF BORIC ACID, FLUOBORIC ACID AND SOLUBLE SALTS OF FLUOBORIC ACID, THE CONCENTRATIONS OF THE SEVERAL INGREDIENTS PER LITER OF WATER CONSISTING ESSENTIALLY AS FOLLOWS: 60 TO 100 GRAMS OF DIBASIC AMMONIUM CITRATE, 8 TO 12 GRAMS OF ANTIMONY POTASSIUM TARTRATE, AND 8 TO 15 GRAMS OF THE COMPOUND SELECTED FROM SAID GROUP. 